Field of the Invention
The invention relates to a method for determining a phase-describing map of a defined region of an object under examination by an imaging magnetic resonance scan, of the type wherein, from this phase-describing map, image data of at least two chemical substance types of the defined region, that differ from one another, such as fat and water, are generated. The invention also concerns a method for determining image data for two different chemical substance types in a defined region of an object under investigation. Furthermore, the invention concerns an image processing device for determining a phase-describing map and, where required, for generating image data of two chemical substances differing from one another of a defined region of the object under examination. The invention also concerns magnetic resonance system with such an image processing device.
Description of the Prior Art
In order to obtain magnetic resonance recordings, i.e. image data generated with a magnetic resonance tomography apparatus, from a region of the interior of the body of an object under investigation, initially the body or the body part to be investigated must be exposed to a static basic magnetic field that is as homogeneous as possible, usually designated as the B0 field. This causes nuclear spins in the body to be aligned parallel to the direction of the B0 field (usually designated the z-direction). Furthermore, with suitable radio-frequency antennae, radio-frequency pulses (RF pulses) are radiated into the object under investigation. The frequency of such RF pulses in the region of the resonance frequency, known as the Larmor frequency, of the nuclei to be excited in the existing magnetic field. These radio-frequency pulses excite the spins of the nuclei, in general hydrogen nuclei, in the object under investigation such that they are deflected by an “excitation flip angle” from their equilibrium position parallel to the basic magnetic field B0. The nuclear spins precess initially around the z-direction and gradually relax again, with the relaxation being dependent on the molecular environment in which the excited nucleus is situated. (de.wikipedia.org/wiki/Relaxation_(NMR) The magnetic resonance signals generated during relaxation are received as “raw data” by radio-frequency receiving antennae. On the basis of the acquired raw data, the magnetic resonance images are subsequently reconstructed. The spatial encoding of the received signals takes place with the use of rapidly switched gradient magnetic fields, which are overlaid on the basic magnetic field during the transmission of the magnetic resonance radio frequency pulses and/or the acquisition of the raw data.
A widely known fundamental problem in the acquisition of the raw data is that the excited nuclei in the body tissue have no uniform resonance frequency in the magnetic field. Rather, the resonance frequency can differ for different tissue types or substance types depending on their chemical environment. This is commonly known as chemical shift. A substance type (or just substance, for short) should be understood herein, in the context of the invention, to be any type of pre-defined chemical substance or any type of atomic or molecular nucleus with particular magnetic resonance behavior. A typical example of different substance types are the substance types fat and water. A substance type may well contain a number of components that have (slightly) different resonance frequencies. For example, as described in greater detail below, the substance type can be described by a chemical spectral model with a number of peaks with regard to the resonance frequency. Thus, the different substance types also cover more complex chemical compounds or mixtures which have different components and possibly different resonance frequencies, but come together into a characteristic spectrum. Particularly relevant in magnetic resonance imaging is the chemical shift of fat tissue in relation to the usually excited water, since fat occurs in many body regions in significant quantities. The chemical shift between fat tissue and water is approximately 3.4 ppm.
There currently exist a variety of methods for generating separate magnetic resonance images of different substance types, such as for generating separate water and fat images. A typical method for achieving this is the “two-point Dixon method”. In that method, raw data are recorded with suitable magnetic resonance sequences during two different echoes, for example, two different gradient echoes or spin echoes. These echoes differ in their echo time so that for one echo, the phase position of the water matches the phase position of the fat, whereas for the second echo the phase position of the water is aligned opposing the phase position of the fat. This is achieved by the echo times being exactly determined suitably in advance, and the magnetic resonance sequences being designed accordingly. Following the signal processing and the usual Fourier transformation for the reconstruction of image data from the raw data, two different types of magnetic resonance image data are obtained therefrom, specifically image data with a matching phase position, the “in-phase image”, and image data with an opposing phase position, the “opposed-phase image”. The signal values in the two images can be described, neglecting the tissue relaxation, as follows:S0(x)=(W(x)+F(x))eiφ0  (1)S1(x)=(W(x)−F(x))ei(φ0+φ)  (2)
In these equations, the water and fat content at a given image point are represented by W(x) and F(x), respectively. S0(x) and S1(x) are the intensity values in the in-phase image and in the opposed-phase image at the respective image point. An image point should be understood, for two-dimensional image data, as a pixel and, for three-dimensional image data, as a voxel. x represents the (for example, also multidimensional) coordinates of the image point. The unit in which the local coordinates are given can simply be defined by the number of image points in the respective direction. The value φ0 denotes the phase in the image which results due to field inhomogeneities and due to the phase during the RF excitation, which can occur in the signal chain and the receiving chain. The phase rotation or phase φ represents a further phase error mainly due to field inhomogeneities arising from eddy current effects in the case of bipolar readout schemas and due to gradient delays, which results between the in-phase echo and the opposed-phase echo. There now exist various algorithms for generating the water image W and the fat image F from the in-phase image and the opposed-phase image using equations (1) and (2). Due to possible field inhomogeneities, gradient delays, eddy currents, etc., it is very important for the two-point Dixon method to determine the global phase rotation φ between the two echo times per image point and then to take it into account in the reconstruction. It is usually also assumed that the variation of the phase rotation is spatially weak, i.e. that the variation between adjacent image points is, for example, <180°.
A significant disadvantage of the conventional two-point Dixon method is the restriction to precisely defined echo times. This significantly reduces the degrees of freedom during the development of suitable magnetic resonance sequences. It is then no longer possible to adapt the echo times to other conditions, in order for example, to develop a particularly fast magnetic resonance sequence to achieve an optimum signal-to-noise ratio, etc.
In the article by Holger Eggers et al. “Dual-Echo Dixon Imaging with Flexible Choice of Echo Times” in Magnetic Resonance in Medicine 65, pages 96 to 107, 2011, a method is described in which the echo times can be selected more flexibly. Herein, as before, however, a relatively simple model is assumed for fat, wherein it is assumed that fat has exactly one resonance frequency line. In fact, however, it is the case that fat and many other substance types have a plurality of resonance frequencies lying close together, i.e. it would actually have to be described by a multi-peak spectral model. In EP 2 431 760 A1, Eggers therefore describes a method in which a multi-peak spectral model for fat can be used, however, so that the whole mathematical description becomes significantly more complex, in contrast to the known classical method. In order, finally, to reach a water image and a fat image, it is therefore proposed in EP 2 431 760 A1, initially to identify all voxels for which a clear mathematical solution exists and subsequently to solve the ambiguity for the other voxels. Herein, the voxels with the unambiguous solutions identified in the direct vicinity are then made use of. In order to achieve this, a correspondingly large number of voxels is required in the images for which such a mathematically unambiguous solution exists. For this purpose, it is shown that it is possible to exert an influence on the number of voxels with unambiguous solutions through a suitable selection of the echo times. It is disadvantageous that if—as distinct from the classical method—the echo times are not exactly defined but that a non-trivial constraint exists regarding the selection of the echo times.
In DE 10 2012 223 789 A1, a method for determining a phase-difference map is described wherein, with the use of the phase-difference map, a phase correction is carried out later to generate the image data of at least two mutually different chemical substance types. The determination of the correct phase values is realized in this method with the aid of a point-by-point progressive growth method, starting from points on the phase-difference map with exact solutions.